Upon making a score in a golf game, a flying distance and a stable directivity of a hit golf ball are regarded as important components. The flying distance of the golf ball is determined by a head speed and momentum efficiently transmitted from a club head to the golf ball at the moment of hitting the golf ball. Additionally, the stable directivity of the hit ball is determined by whether a face part of the club head faces a line direction of a desired play by restricting a covering phenomenon (a phenomenon that a toe-side part advances more than a heel-side part) of the club head at the moment of hitting the golf ball and the club head takes a square posture against the golf ball by restricting a toe-down phenomenon of the club head.
In the past, Japanese Patent Application Laid-Open No. H06-98954 discloses a club head which reduces and restricts a toe-down phenomenon or a covering phenomenon in such a manner that the center of gravity of the head is moved to a heel side and an outline shape of a back part (a back surface of the head) of the club head is formed so that a curvature (curve state) of a heel-side part is larger than that of a toe-side part.
Recently, a study has been proposed in which a shape of the head is designed from viewpoint of fluid mechanics. For example, Japanese Patent Application Laid-Open No. 2002-291947 discloses a club head which improves posture stability in such a manner that a groove is formed in a back part of the club head so that a vortex flow produced in a head surface is hardly separated from the head surface and a vertical and horizontal deviation of the club head is reduced and restricted during a swing action.
Japanese Patent Publication No. H07-4438 discloses a club head which prevents a laminar separation of a laminar boundary layer flowing along a crown part and a sole part and reduces air resistance in such a manner that an upper-lower-end outline of a face part is formed into a curve surface having a curvature radius of 10 mm or less and a small unevenness is formed in the curve surface.
U.S. Pat. No. 5,980,394 discloses a club head which realizes a stable head posture during a swing action in such a manner that a vortex flow producing part having a concave shape is formed in a crown part, a toe-side part, and a sole part.
Japanese Patent Application Laid-Open No. 2001-212267 discloses a club head which maintains and corrects a head posture to be appropriate in such a manner that a toe-side part and a heel-side part are provided with a concave groove opened outward and extending in a longitudinal direction and a high-speed air stream is caught by the concave groove during a downswing action.